Filtering panel and method of making the same

ABSTRACT

The present invention relates to a filtering panel ( 100 ) for a disk filter and a disk filter for water purification, comprising a metal filter cloth ( 16, 116 ) such as a stainless steel filter cloth, pre-tensioned in both warp and weft directions and fastened to a frame ( 10, 110 ) comprising a peripheral portion ( 11, 111 ), which is integral with an inner grillwork ( 12 ) and which divides the inner area of the peripheral portion ( 11, 111 ) into fields ( 15, 115 ). The frame ( 10, 110 ) comprises a plastic material reinforced with glass fibers or carbon fibres such as polypropylene loaded with fibreglass in an amount of at least 20 weight percent. Each field of the grillwork ( 12, 112 ) has an area not exceeding 10% of the inner area of the peripheral portion ( 11, 111 ). The pre-tensioning of the filter cloth ( 16, 116 ) in both directions is from at least 20% of the yield stress of the cloth ( 16, 116 ). Further the present invention relates to a disk filter comprising said filtering panels ( 100 ) and method of making said filtering panel ( 100 ).

FIELD OF TECHNOLOGY

The present invention relates to water purification and in particular afiltering panel used in filtration equipment called disk filter and amethod of making the same. The filtration in a disk filter is obtainedby means of a series of filter disks arranged coaxially andequidistantly.

BACKGROUND

Rotary disk filters comprising a drum having a central longitudinal axisand a plurality of filter disks or disk-shaped filter members beingrotatably arranged about the axis are known from, for example, WO2004/076026 A1 and WO2008/021270, the contents of which are herebyincorporated herein by reference in its entirety. During operation thelongitudinal axis carrying the filter disks is rotated. From theinterior of the drum a filtering liquid is conducted through openingsinto the disk filters.

Each filter disk is composed of a series of equal filter panels,substantially trapezoid-shaped and arranged radially. Each filter panelcomprises a peripheral frame to which is applied a filter net, in thefield called cloth. The filter disks are partially immersed in the waterto be filtered. The filtration takes place from the inside out, that isto say the filtering takes place from the inside of the filter memberand out through the filter cloth.

The liquid level inside the filter is higher than the liquid leveloutside the filter disks. Due to the difference in level between the twofaces of the filter disk, obtained by compartments or bulkheads, whichdifference corresponds to a difference in pressure between the two sidesof the filter cloth, which generates a hydraulic load on the latter andwhich causes the water flowing through the cloths to be filtered.

These disk filters are normally used for the final (called tertiary)filtration in municipal sewage treatment plants, or for the filtrationof water used in the industry when a fine filtration is required (filtercloth with spacing of 10 to 100 microns).

The conventional cloths used in the disk filters are of polyester andare carried by a frame comprising a peripheral portion, substantially inthe shape of a trapezium, all constituting the desired filter panel.Each panel covers a sector of a circular crown of a filter disk of adisk filter). These cloths are made up of thin threads of polyesterwoven according to a warp and weft which create square openings of thespacing desired. The size of the wires, as reduced, generates a netwhose free area takes up between 10 and 15% of the surface of the cloth,with a consequent limitation of the flow of water which is filtered.

The polyester cloths are fixed to the frame, which made made ofpolypropylene, by heating and pressure, after a modest tension of thecloth in both directions, and subsequent cooling. To measure the tensionof a cloth tensiometers are widely used. Tensile measurements or testsare used to determine the tensile strength, yield stress or yieldstrength and other tensile properties. In the following the tensileproperties are expressed as a percentage of the yield stress or denotedas force per unit length of the warp and weft, respectively. Forcharacterizing the tensile properties of filter cloths one importantparameter is the yield stress, which is the stress level at which aplastic material, metal or other material ceases to behave elasticallyand the material begins to deform plastically.

More recently AISI 316 stainless steel cloths have been used withfeatures, in terms of free area or spacing, similar to those of theaforementioned polyester cloths. These cloths, in particular those knownas “plain Dutch weave”, have a warp and a weft which generate apractically three-dimensional structure which the optimisation of thefeatures of permeability in addition to facilitating the cleaning of thecloth, so they give excellent results. These cloths have warp threads ofa greater diameter than weft threads, with fewer warp than weft threads,resulting in a robust and stable cloth. They have, however, anelasticity lower than that of polyester. In particular it has been foundthat when they are stretched and fixed to the frame and used in a diskfilter, in which they are subjected to a perpendicular hydraulic loadwhich is reversed during the periodic phases of so-called backwashing,they are affected by the resulting phenomenon of fatigue, which leadsthem to premature breakage.

Research carried out has established that the parameters which determinethe breakage are:

-   1—the deflection by which the cloth, fixed to the relative frame, is    distorted when subjected to the hydraulic load;-   2—the number of cycles of inversion of the hydraulic load.

One solution adopted to try to extend the life of these stainless steelcloths as far as possible is to stretch them, both in the weft and thewarp directions, almost to their yield limit (normally between 12 and 17N/mm), so as to minimise the deflection when they are subjected tohydraulic load, and then fasten them thus stretched to the relativeframe. In this way, when the cloth is subjected to the alternatehydraulic load, the deflection with which it is distorted is practicallynil and hence not affected by most of the fatigue phenomenon, with theresult of significantly lengthening its useful lifetime, so that itachieves an acceptable duration.

This solution, however, requires the use of particularly rigid frames,which are also made of AISI 316 stainless steel, as well employingmethods of welding the cloth to the frame (once stretched in bothdirections almost up to the yield limit) which are rather sophisticated,so as to avoid the fatigue stress in the vicinity of the areas where thecloth is welded to the frame, which are areas of unavoidable weakness inthis respect. In said area of weakness there is a tendency to be tornaway from the filter frame if the disk filter is in operation.

Another solution which has been adopted and which still uses a stainlesssteel frame, consists of minimising the deflection of the cloth withouttensioning the filter cloth. This is achieved by using a frame ofstainless steel constituted, in addition to the peripheral portion, alsoby a perforated metal sheet inside the frame, which precisely dividesthe area inside the peripheral portion into windows of a much smallersize, resulting in a drastic reduction of the deflection under hydraulicload. Said windows may be manufactured from stainless steel sheets bypressing through a die with the appropriate hole size pattern and havethe disadvantage to reduce the effective filterimg area. In addition tothis reduction of the area, before fixing the filter cloth, a large meshnet is fixed to the stainless steel frame.

The sum of the two devices creates a very solid support for thestainless steel filter cloth, minimising the deflection, and so thefatigue stress, thus only having the necessity to stretch the filtercloth to a minimum to hold it in position during the operation of itsfixing to the outer portion of the frame.

The solution described above results in a filter panel of a rathercomplex structure, in addition to involving the need for the frame alsoto be made of stainless steel, and also requiring a particular weldingprocess of the filter cloth to the peripheral portion of the frame,which makes costs rise considerably. All this, added to the higher costof the stainless steel filter cloth compared to that of the polyestercloth, defeats the economic advantage of the optimisation of thefeatures of permeability and facility of cleaning which are obtained byusing the stainless steel filter cloth.

SUMMARY OF THE INVENTION

The purpose of the present invention is to provide a filtering panel fordisk filters, equipped with a metal cloth such as a stainless steelcloth, which does not present the aforementioned disadvantages of theknown filtering panels shown exemplarily with stainless steel cloth.

Another object of the present invention is to manufacture filteringpanels using metal cloth and avoid the fatigue stress in the vicinity ofthe areas where the metal cloth is attached to the frame. Further asolution is needed to reduce the distortion effect by deflectionoccurring typically when hydraulic load is applied on metal cloths suchas stainless steel.

It is also desirable that the filter frames of disc filters should bemade lighter than stainless steel frames and at the same time staysufficiently sturdy and robust to enable more tensioning than withfiltering panels using conventional polyester filter cloth.

A further object is to provide a filter cloth frame, which hasdimensions of the known filter panels with polyester cloth, having theadvantage of being introducible and suited for replacement in existingfilter frame supports of disk filters for filter panels with polyestercloth.

Another object of the present invention is to provide a filter clothframe which can be manufactured at relative low costs. Yet anotherobject is to provide to make the manufacturing process as, for instance,fastening the metal cloth to a filter frame easier and less costly. Moreparticularly, when cutting the expensive filter cloths such as wovenstainless steel cloths for the filtering panels, there is the drawbackthat large amount of expensive filter cloth are wasted, when usingconventional tensioning techniques. Therefrom derives the purpose toavoid filter cloth waste, when making the filtering panel.

A still further object of the present invention is to provide a toolallowing reliable attachment of a filter cloth to the filter cloth frameand reproducible conditions with regard to tensile properties of thefilter cloth.

These purposes are achieved and the relative technical problems aresolved by the filtering panel according to claim 1, a filtering disk andthe method according to claims as defined by the independent claims.Other features of the filtering panel according to the present inventionare specified in the dependent claims and are discussed in furtherdetail below.

One or more aspect of the disclosure is directed to a filtering panelfor a disk filter for water purification, the filtering panel comprisinga metal filter cloth and a frame comprising a peripheral portion,wherein the frame consists of a plastic material loaded with fibres ofglass or carbon, wherein the peripheral portion is integral with aninner grillwork, which divides the inner area of the peripheral portioninto fields and each field has an area not exceeding 10% of the innerarea of the peripheral portion. Said metal of the filter cloth isselected from the group comprising stainless steel, bronze, copper,brass, titanium, nickel and alloys and the filter cloth is pre-tensionedin both warp and weft directions at least 20% of the yield stress of thecloth and fastened to the frame.

Another aspect of the invention is directed to a filtering panel for adisk filter for water purification, the panel comprising a stainlesssteel filter cloth, pre-tensioned in both warp and weft directions andfastened to a frame comprising a peripheral portion integral with aninner grillwork which divides the inner area of the peripheral portioninto fields, characterised in that:

-   -   the frame consists of polypropylene loaded with fibreglass in an        amount from 20 to 40 weight percent;    -   each field has an area not exceeding 10% of the inner area of        the peripheral portion;    -   the pre-tensioning of the filter cloth in both directions ranges        from 20 to 30% of the yield stress of the cloth.

However it will be understood that the invention is not limited to ametal filter made of stain less steel. Other metal cloths not being madeof stainless steel but of bronze, copper, brass, titanium, nickel andalloys comprising screens can be used. Said metals used as woven filtercloth have an elasticity lower than that of polyester. Hence, they areaffected by the phenomenon of fatigue, which can effectively avoided bypre-tensioning and providing a grillwork as described in the disclosureof the invention.

Moreover, it is understood that the embodiments of the invention are notlimited by the type of plastic material. Alternatively to abovementioned polypropylene (PP) other thermoplastic materials can be used,which are suited to manufacture components by injection molding and canbe reformed or welded by applying heat. The materials of the frame andintegral grillwork can be selected from the group comprisingpolyethylene (PE), polyphenylene oxide (PPO), a blend of PPO and styrene(Noryl), polyphenylene sulfide (PPS) and polyvinyl chloride (PVC) or thelike.

One or more aspects of the disclosure provides for a filtering panel ormethod of upgrading an existing disk filter. Accordingly a filteringpanel, wherein the peripheral portion of the frame has the same size asthe peripheral portion of a panel with a frame of polypropylene or thelike and a polyester filter cloth, may replace the latter in an existingdisk filter.

One or more additional aspects of the disclosure provides for a methodof making a filtering panel, the method comprising:

-   -   injecting plastic material loaded with fibers of glass or carbon        into a mold tool to integrally form a frame with a peripheral        portion and a grillwork    -   locating the cured frame into a tensioning tool;    -   placing a metal filter cloth on top of the frame;    -   fixing the peripheral edge region of the metal filter cloth by        the tensioning tool;    -   pre-tensioning the filter cloth to at least 20% of the yield        stress limit of the metal filter cloth; and    -   applying pressure and heat on the pre-tensioned filter cloth to        fasten the filter cloth on at least a portion of the peripheral        portion by embedding the filter cloth in the plastic material.

The solutions in accordance with the present disclosure provide afiltering panel, disk filter and method for making the filtering panelto significantly increase the lifetime of filtering panels with metalclothing such as for example stainless steel. Another advantage that maybe realized in the manufacturing method is that tensioning can beprovided while minimizing waste of the filtering cloth. No one advantageis critical to the embodiments.

Other advantages of the present disclosure and the invention will bemore readily understood from the following description of exemplaryembodiments and drawings. Any disclosed embodiment may be technicallycombined with any other disclosed embodiment(s).

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying figures illustrate exemplary embodiments of thedisclosure and serve to explain, by way of example the principles of thedisclosure and are not intended to be drawn to scale. The figures areincluded to provide illustration and a further understanding of thevarious aspects and embodiments, but are not intended to restrict thedisclosure to the embodiment illustrated in the figures. Where technicalfeatures in the figures or detailed description are followed byreferences signs, the reference signs have been included for the solepurpose of increasing the intelligibility of the figures anddescription. For purposes of clarity, not every component may be labeledin every figure.

In this description reference is made to the accompanying drawings, inwhich:

FIG. 1 is a front view of the single frame of a filtering panelaccording to the present invention;

FIG. 2 is a view of the filtering panel according to arrow 2 of FIG. 1;

FIG. 3 is a cross-section according to line 3-3 of FIG. 1;

FIG. 4 is a front view, greatly enlarged, of a piece of metal filtercloth, applicable to the frame of FIGS. 1-3 to obtain a filtering panelaccording to the present invention;

FIG. 5 is a view according to arrow 5 of FIG. 4.

FIG. 6 shows a diagrammatic flow chart of a method for making afiltering panel according to an embodiment of the disclosure;

FIG. 7 is a schematic exploded view of a frame of a filtering panelaccording to another embodiment of the disclosure;

FIG. 8 is a schematic illustration of the filter frame of FIG. 7 withthe metal cloth located on the peripheral portion of the filter frame;

FIG. 9 shows a diagrammatic illustration of a finished filtering panelof FIGS. 7 and 8 respectively;

FIG. 10 shows a schematic illustration of a tensioning tool with afilter frame;

FIG. 11 shows the view of the tensioning tool according arrow 310 and across-section along the dashed line shown in FIG. 10; and

FIG. 12 shows diagrammatic illustrations of parts of the tensioning toolof FIG. 11 and a schematic compression device.

DETAILED DESCRIPTION OF THE INVENTION

As can be seen from FIGS. 1-3, frame 10, in the trapezoidal form of thewhole, comprises a peripheral frame 11 within which a grillwork ispresent, indicated as a whole with 12, formed of two series ofperpendicularly intersecting strips, respectively 13 and 14, so as toform a multiplicity of fields 15 which, apart from some of theperipheral fields, are rectangular with the largest dimension which is alittle less than twice that of the smallest. In this specific case thelarger fields have an area which does not exceed 5% of the inner area tothe peripheral portion 11. More generally, it is found that in order tobe sure not to cause the fatigue phenomena illustrated above, the areaof the field must not exceed 10% of the area inside the portion 11.

By using the grillwork constituting of the strips 13 and 14 incombination with the below prescribed pre-tensioning the deflection ofthe metal filter cloth 16, which is, for instance made of stainlesssteel, is significantly reduced, when hydraulic load is applied and thusthe lifetime of the produced filtering panel can be increased. In thisway service requirements for disc filters can be minimised.

The inner perimeter of the peripheral portion 11 of the frame 10 definesthe circumference of the so-called filtering area for the inside-outfiltration process of the filter panel. It is noted that the dimensionsof the strips and corresponding field areas of the rectangulars with thelargest dimension range between, for instance, at least 2.5% and maximum10% of the inner area of the peripheral portion. Constructing thegrillwork with said dimensions the effective filtering area is notsignificantly reduced. Hence, a higher filtration capacity can bemaintained, compared to prior art solutions using stainless steel clothswith perforated sheets and meshes in stainless steel frames.

It is further noted that compared to polyester filter cloths metalfilter cloths such as stainless steel filter cloths enable higheroperating head loss capabilities.

We also note that the transverse strips 13 are considerably more robustthan the longitudinal strips 14, whereby the latter download theappropriate load on the first ones. In any case we have a particularlyrobust frame 10, thanks also to the peripheral portion 11 having anL-shaped cross section. As shown in FIG. 3 the L-shaped cross sectioncomprises a first leg 21, which is in operation located flow parallel,and a second flow-transverse leg 22. Said second leg 22 providesadditional stiffness to the first leg 21. The transverse strips 13 havethe same height in flow direction as the first leg 21 and provide arobust support for the smaller longitudinal strips 14.

The flow-transverse leg 22 provides an inner edge portion 26 forattaching the filter steel cloth 16 (not shown in FIG. 3) to the frame10. Further the circumferential outer edge portion 25 of the second leg22 can be used for providing a further attachment region for the filtercloth 16 as below described in respect to the tensioning tool 300 andcompression device 350 illustrated in FIGS. 10-12.

The grillwork 12 forms a single piece with the peripheral portion 10, apiece which is obtained in a conventional manner by hot molding orinjection molding of a thermoplastic plastic material such aspolypropylene load with about a 30% in weight of glass fibre (forexample that of the company Campiresine S.R.L. of Turate, Como). Thepercentage by weight of glass fibre may in fact vary in a field whichgoes from 20 to 40%.

The less the percentage of glass fibre the lighter the filter frame canbe made, whereas a higher percentage of glass fibre yields a more robustfilter frame and accordingly more pre-tensioning can be applied. Otherfibre materials can be used such as carbon to reinforce the plasticmaterial. Carbon has the advantage to enable a further reduction inweight. Said fillers or others alike can not only be combined withpolypropylene (PP) but also with polyethylene (PE), polyphenylene oxide(PPO), Noryl, polyphenylene sulfide (PPS) and polyvinyl chloride (PVC)and have the object to sufficiently reinforce the plastic frame.

In this way, using for the peripheral portion the same dimensions of theperipheral portion in polypropylene of the known filter panels withpolyester cloth, we get a sufficiently sturdy frame which also has theadvantage of being replaceable in the filter panels with polyester clothin existing disk filters.

The need to have a sufficient sturdy and rigid frame is due to the factthat the said stainless steel cloth when it is fixed to the peripheralportion is previously subjected to a tension in both directions ofbetween 20 and 30% of its yield stress, a value which is higher thanthat to which the conventional polyester filtering cloths applied to theconventional polypropylene frames are subject.

It has been possible to check that the stainless steel filter cloth canbe fixed, after the aforementioned pre-tensioning, to the peripheralportion 11 of the frame 10 (for instance of polypropylene reinforcedwith glass fibre) in a manner similar to that used to fix—precisely byheating and pressure (so that the cloth appears embedded in thepolypropylene of the portion 11) and subsequent cooling—the polyestercloth to the polypropylene frame of the conventional filter panelsdescribed at the beginning, which evidently constitutes a considerablesimplification, with a significant economic advantage.

A filtering panel according to another aspect of the present inventionmay have a filter cloth made of AISI 316L stainless steel with a spacingranging from 5 to 100 micron. In particular a filter cloth which can beused to be fixed to the frame 10 is shown, greatly enlarged, in FIGS.4-6 and indicated by 16. This is of AISI 316L stainless steel,distinguished by the brand Betamesh®, and can have spacings of between 5to 100 microns. A spacing corresponding to the pore-size of, forinstance, 40 μm has a yield stress of typically about 140 N/cm or 14N/mm in both warp and weft direction. From the FIGS. 4-6 it is seen thatthis cloth is composed of warp threads 17 embedded in weft threads 18and 19. This kind of stainless filter cloth possesses good permeability.Obviously other types of stainless steel filter cloths or metals clothsmay be used, provided that they have the appropriate filtering features.Depending on the pore size and woven filter threads used the maximumelastic limit or yield stress may vary in warp and weft directions.

An example for a finished filtering panel obtained by fixing theabove-mentioned metal or stainless steel cloth has been schematicallyrepresented in FIG. 9. The shown cloth 116 is embedded in the plasticmaterial such as polypropylene of the peripheral portion 11. How thispanel is made is described below in more detail and represented in theFIGS. 6 to 11.

The filtering panel according to the present invention combines theadvantage of possessing good permeability features, conferred by the useof stainless steel filter cloths, at a cost decidedly less than that ofthe known filtering panels which already use this type of cloths butfixed to a frame of stainless steel. Furthermore it can be replacedwithout problems in the filter panels currently present in the existingdisk filters, with a life span at least comparable to that of the knownfiltering panels.

Yet to another aspect, the present disclosure is directed to a method 60for making a filtering panel with metal cloth, the method comprising:

-   -   injecting in the first method step 101 plastic material into a        mold tool to integrally form a frame 110 with a peripheral        portion 111 and a grillwork 112;    -   locating (method step 102) the cured frame 110 into a tensioning        tool;    -   placing (method step 103) the steel cloth 116 on top of the        frame 111;    -   fixing (method step 104) the peripheral edge region of the        filter cloth 116 by the tensioning tool 300;    -   pre-tensioning (method step 105) the filter cloth 116 to at        least 20% of the yield stress limit of the cloth 116;    -   applying pressure and heat (method step 106) on the        pre-tensioned filter cloth 116 to fasten the filter cloth on a        portion of the peripheral portion (111) by embedding the filter        cloth in the plastic material.

For simplicity the method step of injecting material into a mold (step101) has not been represented in a figure, since the manufacturingprocess of injection molding for producing thermoplastic plasticmaterial parts such as the integrally formed filter frame 110 comprisinga grillwork 112 is evident for an expert of the technical field.

The heating temperature of a heated compression device 350, whichperforms the method step 106 and is shown in FIG. 12, is chosen suchthat the thermoplastic plastic material of the peripheral portion (111)softens at the region of the at least one bonding and embedding seam,wherein the temperature stays below the melting point of Teflon (PTFE).

Yet another aspect of the method according to the present inventioncomprises:

-   -   cooling (107) the heated plastic material, wherein the plastic        material is selected from the group comprising polypropylene        (PP), polyethylene (PE), polyphenylene oxide (PPO), a blend of        PPO and styrene (Noryl), polyphenylene sulfide (PPS) and        polyvinyl chloride (PVC).

The further method step 107 of cooling of the heated plastic material isperformed for irreversibly fastening the filter cloth 116 to therespective portion or portions of the peripheral portion 111. Aplurality of portions is needed for fastening if for instance two ormore bonding seams 315 instead of one bonding seam are provided on theperipheral portion 111.

FIG. 7 shows schematically an exploded view of the filtering panelassembly 100 comprising the cured filter frame 110, as manufactured inmethod step 101, and a filter cloth 116 made of stainless steel,titanium or the like.

The view of FIG. 7 further indicates potential dimensions of the filterframe 110. As can be seen the peripheral portion 111 includes twosubstantially parallel sides Y1 and Y2 and two non-parallel sides. Thedimensions shown in FIG. 7 are not to scale and may be for example asfollows:

The smaller parallel sides of the trapezoid Y1 and Y2 measure 48 cm and22 cm, respectively. The distance from the outer edges of each of saidparallel sides Y1 and Y2 i.e. in the longitudinal direction of thestrips 114 is about 57 cm. Moreover, in order to achieve the conditionthat each field does not exceed 5% of the inner area of the peripheralportion 111 the exemplary rectangular field area 115 has the sidelengths of A=10 cm and B=6 cm.

The filtering panel according to the exemplary embodiment shown in FIG.7 has a peripheral portion 111 of the frame 110, which has the same sizeas the peripheral portion of a panel with a frame of PP or the like anda polyester filter cloth, so that it can replace the latter in anexisting disk filter. The trapezoidal form and respective dimensions aredesigned to conform with a filter support of a disk filter (not shown),wherein said filter support serves to support the bottom sides Y2 andthe respective non-parallel sides of a pair of filter panels 110 inorder to provide inside-out filtration. A plurality of filter supportsare positioned to secure the filtering panels 110 on a drum to form adisk filter.

Alternatively to polypropylene other thermoplastic materials can beused, which are suited to manufacture components by injection moldingand can be reformed or welded by applying heat. The materials of theframe 110 and integral grillwork 112 can further be selected from thegroup comprising polyethylene (PE), polyphenylene oxide (PPO), a blendof PPO and styrene (Noryl), polyphenylene sulfide (PPS) and polyvinylchloride (PVC) or the like. Said plastic materials can be filled withfibers of glass or carbon for reinforcement.

Yet another aspect of the method according to the disclosure of theinvention is directed to a disc filter (not shown) comprising a drum,which has a central longitudinal axis and which is rotatably arrangedaround the same and is adapted to receive a liquid which is to befiltered, and at least one diskshaped filter member, which on theoutside of the drum extends outwards in the transverse direction of thedrum and consists of releasably secured filtering panels 10, 100,characterized at least by the pre-tensioning of a stainless filter cloth16, 116 by at least 20% as well as by a grillwork 112, wherein eachfield thereof does not exceed 10% of the inner area of the peripheralportion.

The embodiment shown in FIG. 7 comprises a non-continuous projectingridge 161 along the inner edge region 26 of the peripheral portion 111.This ridge can be made continuous or non-continuous. The ridge canoptionally be provided to support the bonding and embedding of thefilter cloth 116 in the plastic material in the method step 106.

FIG. 7 shows schematically the method step 103, wherein the steel cloth116 is placed on the top of the frame as indicated by the arrows 133.

FIG. 8 is a schematic illustration of the filter frame with thestainless filter cloth after the filter cloth is located on the filterframe 110 (see method step 103). Moreover, it can be seen that thedimensions of the stainless filter cloth corresponds to the outercircumference of the peripheral portion 111 of the filter frame 110.This outer circumference is cut after the metal filter cloth isirrecersibly fased by at least one bonding seam.

FIG. 9 shows the filtering panel 100 of FIG. 8 with the at least onebonding seam 315 i.e. after in the method step 106 pressure and heat hasbeen applied. The at least one bonding seam 315 is continuous andlocated above the inner edge region 26 of the peripheral frame portion111. In addition to the bonding seam 315 shown in FIG. 9 a furtherbonding seam may be provided at the outer edge (25) of the peripheralportion 111 (not shown). After cooling the plastic material (in methodstep 107) the at least one bonding seam 315 of the filter cloth 116 isfixedly or irreversibly attached to the inner edge region 26 of theperipheral portion 111.

FIG. 10 shows a schematic illustration of the tensioning tool with afilter frame 110 positioned in the tensioning tool 300. In thisillustration the filter frame 110 is surrounded by four movable elements301, 302, 303 and 304 of the tensioning tool 300. As can be seen thefilter frame 110 is located in the tensioning tool 300 such, that thesecond leg 22 of the peripheral portion 111 faces upwards.

FIG. 11 shows the cross-sectional view of the tensioning tool accordingto arrow 310 indicated in FIG. 10. In this illustration not only thefilter frame 110 is located in the tensioning tool but also the filtercloth 116. The filter cloth has to be cut a few cm larger than the outercircumference of the peripheral portion 111 of the filter panel, inorder to enable a fixation by the tensioning tool 300, more particularlyby the elements 301, 302, 303 and 304, respectively. Cutting the filtercloth in this way avoids unnecessary waste of the expensive clothmaterial. How the fixation works in detail is explained below withregard to FIG. 12.

The cross-sectional views of FIG. 11 and FIG. 12 show that the outeredges of the filter frame 110 are arranged between the tensioningelements 301 and 302. Each of these elements 301 and 302 can bedisplaced horizontally (as indicated by the double arrow H) via thedisplacement means 130. The other opposing elements 303 and 304 are alsocoupled with respective displacement means to enable horizontalmovements for pre-tensioning.

FIG. 12 shows diagrammatic illustrations of parts of the tensioning tool300 and heated compression device 350 as well as an enlargement of thetensioning tool 300. The enlargement shown in the circle of FIG. 12shows a fixation angle 311 of the elements 301 and 302 and at eachelement 301 and 302 a plate 131, which is vertically movable (asindicated by the arrows V) to fix the filter cloth 116 between the downfacing side of the angle 311 and the plate 131. The configuration of theangles 311 and plates 131 at each element 301, 302, 303 and 304 allow tosecurely grip the filter cloth like a forceps.

After the opposing elements 301 and 302 and respective counter elements131 of the tensioning tool 300 have fixed the filter cloth 116 by movingeach plate 131 (see arrow 104) against the corresponding fixation angle311 (see method step 104 as shown in the enlargement of FIG. 12), thefilter cloth can further be pre-tensioned by moving the elements 301 and302 in opposed direction as indicated by the arrows 105. Moving theelements 303 and 304 horizontally in an analogous way (not shown) thefilter cloth 116 can be pre-tensioned in both the warp and weftdirections.

This specifically designed tensioning tool 300 enables an admittedlydifficult tensioning of a filter cloth, which was cut to about the outercircumference of the filter frame 110 with maximum a few cm overlap withrespect to the outer circumference of the peripheral portion 111. Theuse of the tensioning tool 300 has the advantage that the amount offilter cloth 116 to be wasted can be reduced to its minimum, whenmanufacturing the filtering panel 100. Since woven metal cloths asstainless steel cloth 116 are known to be expensive to manufacture, thedesign of the tensioning tool 300 provides a solution to significantlyreduce manufacturing costs, even if the filter cloth needs to betensioned at least 20% of its yield stress.

FIG. 12 shows further a diagrammatic illustration of the method step 106(see arrows), wherein the pre-tensioned filter cloth 116 and the belowlocated plastic material is heated under pressure at least along theinner edge 26 of the peripheral portion 111 by the compression device350. Applying simultaneously pressure and heat (downward directed arrows106) results in fastening the filter cloth by bonding at least a portionof the peripheral portion panel 111. In order to avoid gluing of theheated plastic material to the face of the compression device 350, ateflon or polytetrafluoroethylene (PTFE) layer 351 or teflon strips inheight of the peripheral portion 111 may be provided. In this way the atleast bonding seam 315 as shown in FIG. 9 can be precisely produced. Theheated compression device may be heated such that two or more bondingseams along the peripheral portion 111 are formed. Preferably twobonding seams at each edge (inner 26 and outer 25) are provided. Byusing the tensioning tool 300 in combination with the compression device350 the manufacturing process and thus reproducibility can be improved.

The method for making filtering panels according to the presentinvention using metal cloths such as stainless steel cloth avoidcomplicated welding processes between steel components and result in adevice with less problems of fatigue stress in the vicinity of theareas, where the cloth is attached to the frame. Further, based on thecombination of a precise regulation of pre-tensioning and design of therobust grillwork within the inner area of the peripheral portion of thefilter frame, the distortion effect by deflection occurring whenhydraulic load is applied, can be significantly reduced.

Moreover, the modular design of the disc filter and its filtering panels10, 110 offers flexibility for a broad range of flows. Applications maynot only be municipial tertiary filtration but also the purification ofindustrial waste water, process water filtration or potable waterfiltration and others alike.

This written description uses examples to disclose the invention,including the best mode, and also to enable any person skilled in theart to practice the invention, including making and using any devicesand performing any incorporated methods.

The patentable scope of the invention is defined by the claims, and mayinclude other examples that occur to those skilled in the art. As usedherein, the term “plurality” refers to two or more items or components.

The terms “comprising,” “including,” “having,” and “consisting,” whetherin the written description or the claims and the like, are open-endedterms, i.e., to mean “including but not limited to.” Thus, the use ofsuch terms is meant to encompass the items listed thereafter, andequivalents thereof, as well as additional items. Only the transitionalphrases “consisting of,” are closed or semi-closed transitional phrases,respectively, with respect to the claims. Use of ordinal terms such as“first,” “second,” “third,” and the like in the claims to modify a claimelement does not by itself connote any priority, precedence, or order ofone claim element over another or the temporal order in which acts of amethod are performed, but are used merely as labels to distinguish oneclaim element having a certain name from another element having a samename (but for use of the ordinal term) to distinguish the claimelements.

The invention claimed is:
 1. A filtering panel for a disk filter forwater purification, the filtering panel comprising: a metal filtercloth; and a frame comprising a peripheral portion, the frame consistingof a plastic material loaded with fibers of glass or carbon, the framebeing integral with an inner grillwork which divides an inner area ofthe frame into a plurality of fields, each field of the plurality offields having an area not exceeding 10% of the inner area of the frame,the inner grillwork including longitudinal strips and transverse strips,the peripheral portion comprising a ridge on at least a portion of aninner edge region of the frame configured to support bonding of themetal filter cloth to the frame, and a portion of the metal filter clothbeing embedded in a plastic material of the ridge along the inner edgeregion of the peripheral portion so as to be irreversibly fastened tothe frame; wherein the metal filter cloth is pre-tensioned in both warpand weft directions to at least 20% of a yield stress of the metalfilter cloth and fastened to the frame.
 2. The filtering panel accordingto claim 1, wherein the metal filter cloth is pre-tensioned in bothdirections to a range of from 20% to 30% of the yield stress of themetal filter cloth.
 3. The filtering panel according to one of claim 1or claim 2, wherein the plastic material is selected from polypropylene(PP), polyethylene (PE), polyphenylene oxide (PPO), a blend of PPO andstyrene, polyphenylene sulfide (PPS) and polyvinyl chloride (PVC) andmixtures thereof.
 4. The filtering panel according to claim 3, whereinthe plastic material is polypropylene and the plastic material is loadedwith fiberglass in an amount from at least 20% to 40% weight percent. 5.The filtering panel according to claim 1 or claim 2, wherein each of thefields has an area not exceeding 5% of the inner area of the frame. 6.The filtering panel according to claim 1 or claim 2, wherein aperipheral portion of the frame comprises an L-shaped cross sectionhaving a flow parallel leg and a flow transverse leg substantiallynormal to the flow parallel leg.
 7. The filtering panel according toclaim 6, wherein the flow transverse leg comprises at least one of aninner edge portion for attaching the metal filter cloth to the frame anda circumferential outer edge portion for attaching the metal filtercloth to the frame.
 8. The filtering panel according to claim 1 or claim2, wherein the metal filter cloth comprises stainless steel and whereinthe stainless steel of the metal filter cloth is AISI 316L.
 9. Thefiltering panel according to claim 1 or claim 2, wherein the metalfilter cloth has a pore size ranging from 5 to 100 micron.
 10. Thefiltering panel according to claim 1, wherein the metal filter cloth ispre-tensioned in both warp and weft directions to at least 30% of ayield stress of the filter cloth and fastened to the frame.
 11. Thefiltering panel according to claim 1, wherein a peripheral portion ofthe frame comprises a polytetrafluoroethylene (PTFE) layer.
 12. Thefiltering panel according to claim 1, wherein the metal filter cloth ispre-tensioned in both warp and weft directions to a yield stress ofbetween about 12 N/mm and about 17 N/mm.
 13. The filtering panelaccording to claim 1, wherein the metal filter cloth comprises at leastone bonding seam configured to be fixedly attached to the inner edgeregion of the peripheral portion of the frame.
 14. The filtering panelaccording to claim 1, wherein a largest dimension of the plurality offields is less than twice a smallest dimension of the plurality offields.
 15. The filtering panel according to claim 1, wherein the metalfilter cloth comprises a metal selected from the group consisting ofstainless steel, bronze, copper, brass, titanium, nickel and alloysthereof.
 16. The filtering panel according to claim 1, wherein thetransverse strips have a greater thickness parallel to a direction ofwater flow through the filtering panel than the longitudinal strips. 17.A disk filter comprising: a drum having a central longitudinal axis,rotatably arranged around the central longitudinal axis and adapted toreceive a liquid to be filtered, and at least one disk-shaped filtermember on an outside of the drum extending outwards in a transversedirection of the drum and including releasably secured filtering panels,wherein each of the filtering panels is a filtering panel as claimed inclaim 1 or claim
 2. 18. The disk filter according to claim 17, whereinthe filtering panels are arranged to perform inside-out filtration ofthe liquid to be filtered.
 19. The disk filter according to claim 17,wherein the liquid to be filtered comprises municipal wastewater. 20.The disk filter according to claim 17, wherein the liquid to be filteredcomprises at least one of industrial wastewater, process water, andpotable water.